“Runner’s stomach” is the common term for exercise-induced gastrointestinal distress (EIGID), a frequent complaint among endurance athletes. This condition involves a range of uncomfortable digestive symptoms that occur during or immediately following intense or prolonged physical activity. EIGID can significantly impair performance and training quality for runners, cyclists, and triathletes. The prevalence of these symptoms is high, affecting up to 90% of athletes in some endurance events. Understanding the underlying mechanisms is the first step toward managing this disruptive phenomenon.
Identifying the Signs
The symptoms of EIGID manifest across the digestive tract and are broadly categorized into issues affecting the upper and lower gastrointestinal regions. Upper GI symptoms typically involve the stomach and esophagus, causing sensations like nausea, vomiting, and heartburn. Runners may also experience epigastric pain or belching, which can be disruptive to breathing rhythm during a sustained effort.
Lower GI issues are often more severe and include abdominal cramping, flatulence, and the sudden, urgent need to defecate. Diarrhea, sometimes referred to as “runner’s trots,” is a common and debilitating symptom that may lead to early termination of a run.
The Root Causes During Exercise
The primary physiological driver of EIGID is the body’s automatic response to strenuous exercise, which involves the redistribution of blood flow. As physical intensity increases, the body prioritizes sending oxygenated blood to the working muscles and skin, severely limiting circulation to the digestive system. This diversion of blood from the abdominal organs is known as splanchnic hypoperfusion.
This reduced blood flow can decrease by 60% to 80% during intense exercise, leading to temporary ischemia, or oxygen deprivation, within the intestinal lining. Ischemia damages the mucosal barrier, increasing intestinal permeability and potentially leading to the leakage of bacteria and toxins into the bloodstream, which exacerbates inflammation and discomfort. The sympathetic nervous system also becomes highly active, releasing hormones that alter gut motility and slow gastric emptying.
Mechanical factors also play a substantial role, particularly in high-impact activities like running. The repetitive, jarring motion of the torso physically jostles the internal organs, leading to trauma to the digestive tract walls and increasing the urge for bowel movements. Runners often experience more symptoms than cyclists due to this vertical impact, which directly contributes to lower GI issues like cramping and diarrhea.
Nutritional Triggers and Mistakes
Specific nutritional choices can significantly aggravate EIGID. Ingesting foods high in fiber or fat before a run is a common mistake because these macronutrients slow the rate of gastric emptying. This delayed transit time means food remains in the stomach and intestines longer, increasing the likelihood of discomfort when combined with reduced blood flow.
Highly concentrated sugar solutions, often found in sports drinks or gels, can also act as triggers. Solutions with an osmolality higher than that of body fluids are hypertonic, and they draw water into the intestinal lumen to equalize the concentration. This influx of fluid increases the osmotic load in the gut, which can lead to bloating, cramping, and an accelerated need to defecate. Excessive intake of fructose, especially without an equal amount of glucose, can be poorly absorbed and exacerbate symptoms.
The use of non-steroidal anti-inflammatory drugs (NSAIDs), such as ibuprofen, is another factor that can worsen digestive distress. These medications increase intestinal permeability, compounding the damage caused by exercise-induced ischemia. Taking NSAIDs before or during a long run heightens the risk of developing more severe symptoms, including gastric bleeding.
Practical Prevention and Management
Preventing EIGID requires a systematic approach that addresses both physiological and nutritional triggers. One effective long-term strategy is “training the gut,” which involves gradually increasing carbohydrate intake during training sessions to improve the body’s tolerance for fuel and fluid. Starting with small amounts and building up over weeks helps the digestive system adapt by upregulating nutrient transporters.
The timing of meals before exercise is also important, with a large meal ideally consumed three to four hours before a long or intense run. Pre-exercise meals should focus on low-residue, easily digestible carbohydrates and be low in fat, protein, and fiber to minimize the load on the gut. For hydration during exercise, utilizing isotonic or hypotonic solutions helps promote rapid fluid absorption without causing excessive fluid shifts in the intestine.
Adopting a low-residue diet in the 24 to 48 hours before events can be beneficial. This strategy involves temporarily reducing foods high in insoluble fiber, like raw vegetables and whole grains, to decrease the amount of solid waste in the lower GI tract. If symptoms arise during a run, slowing the pace can help, as reducing exercise intensity lessens the degree of splanchnic hypoperfusion, allowing the gut to recover slightly.